The invention relates to a method and a system for digital signal transmission particularly in mobile telephone systems. More precisely the invention relates to transmitting channel encoded signal using a method like macro diversity.
One of the major problems in mobile telephone systems is to efficiently use a limited radio resource and to simultaneously offer good quality for the user of the system. The problem becomes more serious as new systems are created where new services requiring fairly high data transmission rates, for example data transmission, are transmitted in addition to speech.
A solution to the quality problem is to use antenna diversity in transmission. This means that the same signal is sent in a base station to a user equipment using two or more different antennas.
The use of various block and convolutional codes in order to minimize transmission errors is commonly known in the art. Coding refers to a signal that is encoded with a coding method so that errors occurring in the signal on the transmission channel can be detected and even corrected in reception. Known codes include parallel concatenated convolutional codes, or turbo codes. The turbo codes are described in the publication Sklar, Bernard: A Primer on Turbo Code Concepts, in IEEE Communications Magazine, December 1997 Vol 35 No. 12, which is incorporated herein as reference. Another known coding mode is space/time coding described in publication WO 97/41670.
The use of macro diversity is also known to those skilled in the art and is described, for example, in U.S. Pat. No. 5,101,501. The downlink macro diversity refers to at least two different base stations which concurrently send the same encoded signal to the user equipment. Interference and fading in one link cannot prevent the signal from arriving at the user equipment as the signal is sent from two different locations. The received signals are connected in the user equipment and the coding employed is decoded, whereby eventual transmission errors can be corrected. The downlink macro diversity is a compulsory property, for example, in the IS-95 radio system.
A disadvantage with macro diversity is that it uses more of the radio resources in the system than a standard connection establishment mode. For example, when two base stations are used, both base stations employ the capacity of a single transmission channel for transmitting the signal to the user equipment. The transmission channel is formed in various ways in different multiple access methods, for example in systems using TDMA (Time Division Multiple Access) the transmission channel is determined as a combination of frequency and time slot. Correspondingly in systems using CDMA (Code Division Multiple Access) the transmission channel is a spreading code. Different CDMA/TDMA hybrid systems are also possible.
It is an object of the invention to provide a method and an apparatus implementing the method so as to solve the above problems. This is achieved with the method described below, the method for digital signal transmission from a radio network subsystem to a user equipment in a mobile telephone system comprising the following steps of: encoding a signal in order to minimize transmission errors on a transmission channel, sending the encoded signal through at least two different base stations, receiving the encoded signal in a user equipment and decoding the received signal coding, dividing the encoded signal into separate signal parts having at least partly different contents, indicating a specific signal part for each base station, the base stations sending their specific signal parts and combining the information of the signal parts received in reception.
The invention also relates to a system for digital signal transmission comprising: a radio network subsystem including a base station controller and at least two base stations, a channel encoder for coding a signal in order to minimize transmission errors on a transmission channel, at least two different base stations for sending the encoded signal, a user equipment for receiving the encoded signal, and a channel decoder for decoding the received signal coding. The system also comprises: a divider dividing the encoded signal into separate signal parts having at least partly different contents, a radio network subsystem being arranged to indicate a specific signal part for each base station, the base stations being arranged to send their specific signal parts and a combiner combining the information in the received signal parts.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the fact that the radio resources of the system are not wasted by sending an entire encoded signal using macro diversity, instead only parts of the signal are sent through different base stations. This allows to achieve an equal result compared to standard macro diversity on account of the efficient coding.
Several advantages are achieved with the method and system of the invention. The reliability of the transmission is nearly as good as when using standard macro diversity but less radio resources are needed in the system. The available resources can be distributed if needed to other users or services.